What would IRS do to verify my return if my dependent is victim of tax identity theft?

Someone filed a tax return using my son's SSN and we got a letter from IRS to clarify it.  My son is a dependent on my tax return and therefore my e-filing was rejected this year.  Now I am mailing in my tax return.  Do you know what steps IRS will take to verify my tax return.  I am thinking we may need to visit the local tax office in person to get it clear out.

Thanks in advance!